I came across user surajramnani, who submits all his posts from the same site (which is completely full of advertising). I understand that aspiring bloggers want to get as much traffic to their site as possible, but I think this is an example of someone who doesn't actually want to contribute the community, and instead wants to pollute it for their own benefit.
I'd be interested to see what other people think about this, and whether we need to introduce a "report spam" button or other method of penalising users like this.